Note: VMware doesn't support ESXi 5.5 running on DL360-G5(in this case Intel Xeon 5150). But it runs smoothly without any issues. Even is not VMware supported in the HCL.
When I was installing ESXi 5.5 (HP-5.73.21 build) on this server I received a purple screen with this warning:
VMware ESXi 5.5 (VMKernel Release Build 1623387)
The system has found a problem on your machine and cannot continue.
VMware ESXi requires the Execute Disable/No Execute CPU feature to be enabled
This means that you server have the No-Execute Memory Protection option disabled.
You can check here VMware KB regarding this issue.
Is very easy to fix it.
Restart you server, enter BIOS and change the option No-Execute Memory Protection to enable.
Then you can re-run ESXi install again.